Flexible borehole liners are in commercial use for sealing subsurface boreholes and providing isolation for discrete spatial resolution of ground water sampling. U.S. Pat. Nos. 5,176,207 and 5,725,055, the teachings of which are incorporated herein by reference, are helpful examples of such subsurface borehole flexible liner apparatuses and methods. Useful background reference may also be had to U.S. Pat. No. 5,176,207, and U.S. Pat. No. 6,283,209 (the entire disclosures of which also are incorporated by reference herein) for further understanding of the use of flexible liners, placed by eversion, to accomplish down-hole pursuits. According to known practice, the interior pressure of the liner is maintained to be greater than the pore pressure in the medium adjacent to the borehole, thus forcing the liner fabric against the borehole wall to achieve a seal of the borehole against flow into and along the borehole hole, relative to the adjacent medium.
However, a concern remains with the function of the borehole liner as a sealing device over very long time periods. That concern is related to the molecular diffusion process, which is well known, to allow the relatively slow transport of contaminants, in solution, through thin barriers from a high concentration volume into an adjacent lower concentration volume. Such a diffusion process is central to the function of U.S. Pat. No. 5,804,743. In a like manner, tri-chloro-ethylene, and similar contaminants of ground water, can migrate via the diffusion process from the ground water external to a borehole, through the flexible liner material, and into the interior of the fluid filled liner which is intended to seal the borehole against contamination migration. In time, the diffusion process may also propagate the contaminant along the liner's axial length to other locations interior to the fluid-filled liner. Thereafter, further diffusion out of the liner interior into the surrounding medium may contaminate the pore fluids of the medium outside the liner, at a location remote from the point or points of diffusion into the liner. This particular diffusion transport path, i.e., (1) into the liner, (2) along the liner, and then (3) out of the liner, can deleteriously bypass the seal against contaminant transport otherwise expected to be provided by the pressurized liner.
The method and system of the present disclosure serve several functions. One is related to a prevention of the diffusion process described above. A second function, realized in actual use of certain embodiments, is to provide a lubricating surface on the liner exterior that reduces the friction associated with the eversion of the liner into position in a borehole. A third advantage is to prevent the contamination of the surrounding pore fluids by diffusion of contaminants entrained in some flexible liner materials during their manufacture. Prevention of diffusion from the liner itself allows the use of liner materials which would otherwise conflict with the pore fluid sample integrity (as representative of the ambient pore fluid).